A|2||Global Positioning System antenna|365|397|45,147,266,162,
The Global Positioning System is based on a network of 21 military satellites in circular orbits 20,200 kilometers above the earth. These satellites broadcast signals that allow the GPS receiver aboard TOPEX/Poseidon to calculate the spacecraft's position in orbit. With augmented ground processing, this position is accurate to within 3 centimeters.
The Global Positioning System antenna shown here is connected to a GPS Demonstration Receiver housed within the Instrument Module. The GPS receiver was included as an experiment because its use in Precision Orbit Determination for a spacecraft had never been demonstrated.
The GPS receiver aboard TOPEX/Poseidon normally does not require commanding or special attention from the ground, and is always tracking. In the event that the receiver loses contact with the GPS satellites, ground-initiated commands are necessary to restart the on-board processor to enable it to re-acquire the GPS signal.
